Reduction of 4,6-bisphenylazoresorcinol is generally carried out with hydrogen by using a noble metal catalyst (the Japanese Patent Open-laid Publication No. Hei 7-242604 official gazette). According to the Japanese Patent Open-laid Publication No. Hei 7-242604 official gazette, the reduction is carried out under a neutral condition in a solvent. As to the solvent, there is described that water, lower alcohols, aromatic hydrocarbons, halogen-substituted benzenes, halogenated aliphatic hydrocarbons, ethers, lower ketones are used alone or in combination. Furthermore, as the post-treatment after the end of the reduction reaction, two methods are described. In a first method, an acid is added and an acid salt is formed after filtration through Celite still under a nitrogen atmosphere to obtain 4,6-diaminoresorcinol, which is filtered off the catalyst and Celite, thereafter purified and isolated. In a second method, an acid is added to the reaction solution to make a salt, which is dissolved, filtered off the catalyst, thereafter purified and isolated.
The former has such effects that recovery of aniline produced in the reaction is easy and that the purification procedure is simple, but filterability is very slow. A yield is low such as 60% due to the loss of 4,6-diaminoresorcinol into the filtrate, thus the method cannot be said as an industrial method.
The latter has high stability in air and good operability since an acid is added to make a salt. Furthermore, filterability is high, which seldom cause any problem. But, it has such a disadvantage that separation and purification of a by-product, aniline, and 4,6-diaminoresorcinol become very complicated due to the presence thereof in an acidic solution.
In the reduction of 4,6-bis(substituted)phenylazoresorcinol, 2 equivalents of an aniline derivative is by-produced in addition to 4,6-diaminoresorcinol. It is the largest problem in the reduction process of 4,6-bis(substituted)phenylazoresorcinol how simply the aniline derivative can be separated with a few loss of an objective 4,6-diaminoresorcinol produced.